It is well known that the quality of whole picked fresh fruit deteriorates rapidly at ambient temperatures. The deterioration rate can be slowed, thereby enabling the fruit to be retained for longer shelf-life, when the fruit is maintained at refrigerated storage temperatures. In most cases, the shelf-life of fresh, ripe whole fruit at refrigerated temperatures (1.degree. to 13.degree. C.) ranges from about three days to six weeks, depending in part on fruit type. Many fruits are picked and packed in the unripe stage in order to lengthen their shelf-life at both ambient and refrigerated temperatures. Unfortunately, with this practice, the fruit frequently lacks or fails to develop optimum ripe-fruit flavour, texture and colour. Fruits harvested at the peak of ripeness possess high quality attributes which are strongly desired by consumers and thus fetch a higher price than fruit picked at the pre-ripe stage of maturity. Thus, the development or discovery of a method to capture and retain the high quality attributes of ripe fruits for prolonged storage periods would be extremely advantageous.
If all whole picked ripe fruits could be treated as one and stored under refrigeration without regard to any specific temperature requirement, then the storability and distribution of the fruits would be simple. Many types of fruits could, for example, be held in one constant temperature refrigerated room, or could be transported in one refrigerated truck at a common refrigeration temperature.
Regrettably, the situation is not that simple. Each species of whole fresh fruit must be stored within a specific custom temperature range so that acceptable quality for that particular fruit type can be retained. For example, the required storage temperature range for bananas is from 12.degree. to 13.degree. C. At temperatures below 12.degree. C., chilling injury to the whole banana occurs and the edible internal tissue and the peel of the banana turn brown or black. Some other whole fruits which are also susceptible to chilling injury are mango, melon, papaya, pineapple and tomato. The chilling injury symptoms for these whole fruits are brown flesh, water-logging of flesh, surface pitting and tissue softening. As a general rule, this latter group of fruits must be stored at temperatures between 7.degree. and 12.degree. C. to prevent chilling injury. In comparison, recommended storage temperature ranges for raspberries, strawberries, peaches, apples and apricots are from -1.degree. to 4.degree. C.
In addition to a temperature factor, the composition of gases in the storage atmosphere enveloping the fruit can influence and prolong the storage life of whole fresh fruits. In particular, a moderately high level of carbon dioxide (2 to 10%) and reduced levels of oxygen (2 to 16%) in controlled atmosphere storage can significantly increase the shelf life of certain fruits such as apples, pears, strawberries and bananas. There is evidence t1at higher carbon dioxide levels and lower oxygen levels advantageously lower the respiration and ripening rates of the whole fruit. On the negative side, however, undesirable physiological disorders and deterioration in quality of the fruit may occur. Indeed, strong anaerobic off-flavour in the fruits may develop.
Apples are a fruit that can be stored successfully for several months at refrigerated temperatures in a specific gas atmosphere without experiencing a dramatic loss in quality. Controlled levels of carbon dioxide, oxygen and humidity in the atmosphere of rooms used for storing whole apples has been a technique which has been successfully used commercially for about thirty years. This procedure is commonly called controlled atmosphere storage. The carbon dioxide level is controlled to specified limits by passing the storage atmosphere gases through an alkali scrubber or hydrated lime. The oxygen is reduced and controlled by an external burner. Using these techniques, apples can be stored for about six to seven months in atmospheres of 2 to 3% carbon dioxide and 2 to 3% oxygen at temperatures around 3.degree. C. without a dramatic downturn in quality.
It has also been determined that perishable produce such as lettuce, celery, and the like, held in transportable containers can withstand long distance transport when the containers are flushed with nitrogen and/or carbon dioxide gas to bring about low oxygen levels. Low oxygen levels have been found to reduce the respiration rate of the produce and retard deterioration. Fruit commodities require at least 1% oxygen to prevent unsightly discolouration and off-flavour development and to permit the ripening of green picked fruit.
Modified atmosphere packaging is the term commonly used for the storage of food in a flexible or semi-flexible bag or pouch with an internal atmosphere which is not controlled but may indeed vary in composition during storage of the commodity due to gas transmission through the walls of the bag or pouch. Plastic films have been used to cover whole fruits in containers so that the atmosphere enveloping the fruit can be modified. Polyethylene box liners, either sealed or unsealed, have been employed commercially for some time for the storage and transportation of apples and pears. It has been found that the respiration of the whole fruit in a sealed, air containing, polyethylene bag will cause a rise in carbon dioxide level and a corresponding reduction in oxygen content in the bag interior. If the temperature of the bagged whole fruit rises above 5.degree. C., it has been determined that the respiration rate of the fruit increases markedly and undesirably high levels of carbon dioxide may be formed even though the polyethylene bag has comparatively high gas permeability. High levels of carbon dioxide (5% or higher) have been found to be harmful because they can cause unsightly fruit discolouration and "off-flavour" development. This is due to carbon dioxide toxicity. To reduce the risk of carbon dioxide toxicity to the whole fruit, the bags are either unsealed or perforated to permit atmosphere exchange, or packets of fresh hydrated lime (which reacts with the carbon dioxide to reduce its level) are placed in the bag prior to sealing.
Literature relating to modified atmosphere packaging of fruit and patents granted for such techniques have conventionally focussed on whole, uncut commodities. In general, these techniques are concerned with the maintenance of a suitable oxygen-containing environment around the exterior of whole fruits in plastic film packages These patented techniques and teachings require that aerobic conditions be maintained in the atmosphere enveloping the whole fruit in order to prevent the development of anaerobic or fermentation "off-flavour" in the fruit. Yet at the same time, the atmosphere must contain a sufficient amount of carbon dioxide and/or nitrogen to inhibit fruit deterioration. Some patents are concerned with the removal of some of the ethylene and carbon dioxide in the surrounding atmosphere by the inclusion of an adsorption packet of chemicals in the package prior to sealing.
In a general sense, the development of modified atmosphere packaging of fruit is in its infancy Studies have focussed on the O.sub.2 /CO.sub.2 gas flushing of the packages containing whole strawberries. It has been determined that with this process, the storage life of this fruit can be lengthened by up to eight days It has also been discovered that when apples are packaged in Cryovac.TM. bags with a modified atmosphere (Cryovac.TM. is a trademark for a thermoplastic packaging film material, the shelf life of the whole apples is extended from one week to four to six weeks (LaBell, Food Processing, January, 152, 1985).
A wide variety of plastic films with different permeabilities to gases are commercially available for the modified atmosphere storage of fruits. Saguy and Mannheim (Cooling and Ripening of Fruits in Relation to Quality, Refrigeration Science and Technology, 149, 1973, Int. Inst. Refrig.) have shown that selected plastic films with various O.sub.2 permeabilities can prolong the shelf life of strawberries. Marcellin (Rev Gen. Froid 64:217, 1974}discussed the use of polyethylene and a silicone membrane for the modified atmosphere storage of several fruits. It should be noted that O.sub.2 transport through the plastic films ensured aerobic respiration of the fruits, yet contributed to the proliferation of aerobic microorganisms such as molds. Modified Atmosphere (MA) packaging, in the accepted sense, involves the evacuation of part or all of the air from the package and the introduction of a mixture of gases. Packages, for MA storage include pallet bags packs (CSIRO Food bag-in-bin, bag-in-box, and portion packs (CSIRO Food Res. Q. 44, 25, 1984).
In summary, controlled atmosphere storage and modified atmosphere packaging of some whole, fresh picked fruits can be used, as an adjunct to temperature control, to prolong the useful storage life for such whole fruits for a few days to a few weeks longer than is possible in non-controlled or modified conditions. Experiments have demonstrated that with some whole fruits, even controlled gas treatments have not been beneficial.
Apart from detracting from the possibility of providing premium ripe fruit and produce to meet the taste of the consumer, the lack of suitable preservation methods for fruit has the following adverse economic impact upon a nation: PA1 1. The importation of fresh fruits into a nation represents a drain on the economic resources of that nation. PA1 2. The potential for export of fresh fruit from one nation to another is limited because of short storage life of the fruit, thereby representing a loss in potential revenue for the nation from external sources. PA1 3. The spoilage rates for fresh fruit are generally in the neighbourhood of 10 to 20% of the total fruit crop, thereby representing an unnecessary waste. PA1 4. The inability to store fresh ripe fruits for long periods means that a considerable amount of fresh fruit is preserved by energy intensive, costly canning and freezing procedures. PA1 1. When edible tissue is separated physically from inedible tissue, and the edible tissue is cut, physical stress incurred in the fruit brings about changes in the metabolic processes of the fruit pieces and the disruption of membrane structures of the edible tissues. Such changes could be responsible for the inhibition or prevention of undesirable physiological disorders and chilling injury in the fruit pieces. Further, the incurred stress may bring about in the fruit pieces certain defence mechanisms against microbial growth. PA1 2. Inedible tissues of whole fresh fruits have been found to be the prime locations for experiencing extensive physiological disorders and chilling injury in the fruit. Thus removal of the injury-vulnerable inedible tissues should provide better overall appearance and colour compared to whole fresh fruit, and thus render the marketable fruit pieces more acceptable to consumers. PA1 3. The removal of inedible tissue from edible parenchyma tissue is believed to obviate any translocation from the inedible tissue to the edible tissue of precursors of the undesirable melanin browning reaction which is common in many fruits. Thus discolouration of the edible tissues should be minimal. PA1 4. Cutting of the edible parenchyma tissue into pieces increases the surface area to volume ratio of the fruit and, as a consequence, it is believed that this assists the oxygen in the gas mixture of importance in the fruit preservation procedure to migrate into the centre of edible tissue pieces at a greater rate and within a shorter time period, and subsequently in situ carbon dioxide would be produced in all cells of the fruit pieces. Further, a gas equilibrium would be reached more rapidly in the edible parenchyma tissue and the surrounding environment.
The inventors are aware of the following references and patents which are more or less pertinent to this invention:
______________________________________ Inventor Issue Date ______________________________________ U.S. Pat. No. 3,111,412 Mouk Nov. 19, 1963 4,001,443 Anantray Jan. 4, 1977 4,006,257 Kolk Feb. 1, 1977 4,055,931 Myers Nov. 1, 1977 4,079,152 Bedrosian et al. Mar. 14, 1978 4,235,750 Cazalet Nov. 25, 1980 4,331,693 Gozdziewicz et al. May 25, 1982 4,337,276 Nakamura et al. June 29, 1982 4,411,921 Woodruff Oct. 25, 1983 4,423,080 Bedrosian et al. Dec. 27, 1983 4,515,266 Myers May 7, 1985 West German Pat. No 2,922,145 Kurz Mar. 20, 1980 3,136,622 Chekalov et al. May 30, 1984 Austrian Pat. No 225,346 Fetkenheue July 31, 1985 ______________________________________
Of these, the following patents brief comment on the basis that they are considered to be reasonably pertinent to the subject invention.
Mouk discloses a method of packaging perishable comestibles including fish, meat, poultry and vegetables for shipment under refrigeration. The method comprises providing a substantially rigid transportation carton, fitting within the carton a composite flexible wrapper having a continuous water resistant, moisture-vapour impermeable film lying adjacent the interior of the carton and a bonded substantially uncompressed, highly water-absorbent batt of cellulosic fibers, introducing refrigerated comestibles into the wrapper, wetting the batt with water, folding the wrapper to overlap itself and completely enclosing the comestibles and closing the carton.
According to Kolk, fruit is prepared for refrigerated storage by cutting the fruit into convenient pieces, and then subjecting the fruit to soaking under vacuum in a solution containing sodium bisulfite or sodium sulfite and citric acid in definite ranges. These ranges are preferably inversely related in a substantially straight-line relationship, so that the high end of one range is used in conjunction with the low end of the other. The fruit pieces are also preferably immersed in a preliminary holding solution containing sodium bisulfite or sodium sulfite and table salt prior to the soaking. Fruit prepared in this way can be held indefinitely under ordinary refrigeration without freezing. The process is particularly suitable for apples.
Woodruff teaches a process for inhibiting the growth of fungi on fresh whole fruits and fresh whole vegetables other than leafy and head vegetables by maintaining the surrounding gaseous atmosphere, which consisted of carbon dioxide in an amount from zero to about 20 percent by volume, molecular oxygen in an amount of about 1 percent to about 20 percent by volume, carbon monoxide in an amount of about 3 percent to about 25 percent by volume, with the remainder being substantially all molecular nitrogen, at a temperature in the range of about 29.degree. F. to about 60.degree. F. for a time sufficient to inhibit growth of fungi on the fresh whole fruits and fresh whole vegetables. Oxygen and carbon monoxide must be maintained in the surrounding atmosphere during the entire storage period.
Bedrosian (U.S. Pat. No. 4,423,080) discloses a controlled atmosphere package for retarding the ripening rate of fruits and vegetables. It is formed from the combination of a sealed enclosure and a packet for the package interior which contains chemical agents capable of absorbing moisture and carbon dioxide from the package environment so as to prevent mold growth or other respiratory injury to the product. Absorption of carbon dioxide and water vapour lowers the pressure inside the package and produces a driving force which allows the entry of sufficient air into the package from the surrounding ambient atmosphere to prevent anaerobic respiration and low oxygen injury to the produce.
Myers (U.S. Pat. No. 4,515,266) discloses a package for preserving whole produce in a wholesome condition for an extended period of time. The package is formed by a sealed container enveloping the produce and filled with a preservative gas (no oxygen) which inhibits bacterial growth. The container is formed by a high barrier film which is so perforated as to assure gas outflow through the perforations from the container to prevent its distortion.